The purpose of an Inertial Navigation Systems (INS) is to compute and provide a navigation solution to a platform on which it is installed. A navigation solution consists of the Position, Velocity and Attitude (PVA) of the INS with respect to the Earth. An INS computes a navigation solution based on the dead-reckoning principle: given the initial position, velocity and attitude of the platform (referred to as “initial conditions”), continuous readings from the sensors of the INS are used to keep an updated navigation solution even and particularly when the platform is in a dynamical state. Two different classes of Inertial Navigation System essentially exist: Inertial platform and Strap-Down.
An “inertial platform” navigation system consists of a gimbaled sensor assembly (hereinafter, “inertial platform” or simply “platform”), including angular rate gyros (or free gyros) and accelerometers. Using the measurements sensed by the sensor and actuators mounted on the gimbals, the platform is maintained leveled and pointed to the north during the whole operation. The azimuth is maintained by an adjacent gimbaled high quality mechanical free gyro that points to the north via a mechanical process known as mechanical gyro compassing. Inertial platforms INS are usually large, heavy, very expensive and highly accurate and mostly found in earlier applications, or in applications where extremely high accuracy is required.
On the other hand, a Strap-Down (SD) INS, sometimes referred to in the literature as analytic platform, normally comprises of at least three angular rate sensors (usually rate gyros) and three accelerometers that provide angular and linear measurements of the dynamics of the INS frame. The sensor assembly is known in the art as Inertial Measurement Unit (IMU) or Inertial Sensors Unit (ISU). Algorithms that are provided at a computation unit of the INS utilize the measurements of the gyros and accelerometers to continuously provide a navigation solution—PVA. Modern inertial navigation systems are mainly of the Strap-Down type. The present invention is applied to the SD class of INS, and hence for the sake of simplicity, from this point on the term INS would in fact be referred to SD INS.
Typical modern gyroscope types Inertial Navigation Systems include MEMS (Micro Electro Mechanical Systems), FOG (Fiber Optic Gyroscope), RLG (Ring Laser Gyroscope) and DTG (Dynamically Tuned Gyro). One can identify several grades in the INS market: low or commercial grade (small and lightweight systems), tactical grade (as in small Unmanned Air Vehicles), navigation grade (as in aircrafts) and strategic grade (as in ships & submarines). Usually the grade of an INS depends on the grade of its gyros.
Higher grades INS, under certain circumstances, are capable of autonomously calculating their initial conditions (true heading, pitch and roll) by means of a process known as Gyro-Compassing (GC). The process of gyro-compassing requires static conditions, during which the sensitive gyros and accelerometers of the INS measure the Earth gravity and rotation vectors, and given these measurements, an algorithm within the INS determines the three angles of the INS with respect to the Earth with accuracy which depends on the quality of the IMU components (gyros and sensors).
Gyro-compassing can also be implemented under dynamic conditions assuming that an external source of information about the platform dynamics is provided. This process is known in the literature as transfer-alignment or in-motion-alignment. The invention disclosed herein, however, deals only with gyro-compassing under static conditions, and so this condition is assumed hereinafter.
Once said initial conditions are determined, the IMU can be used to track dynamic changes so as to provide continuous navigation data. Since all the INS components are prone to some errors, once the system is no longer stationary, and changes are measured relative to the initial state, the inaccuracy of the updated position data increases with time as a function of the inaccuracy in the components and, to a lesser extent, the trajectory. In modern inertial navigation systems, external references, such as GPS measurements, are often used to limit the maximum total error to some constant values that depend on the quality of the additional equipment.
The initial conditions may also be provided to the INS from an external source, such as a stage based north finding system (NFS). A stage based NFS typically comprises at least one gyro and at least one accelerometer that are mounted on a rotatable stage. The stage is a leveled platform whose orientation may be changed about one axis. The stage axis of rotation is vertical—“Z axis” similar to “Z” axis demonstrated in FIG. 1.
Typically, the north finding measurement in the NFS is performed by two gyros and two accelerometers at two separate orientations of the stage, e.g., 0° and 180°. Typically, the gyros and accelerometers used for NF are aligned with the stage horizontal axes (X and Y similar to the respective axes demonstrated in the FIG. 1). Measurement errors due to components inaccuracies that occur in a single orientation measurement (e.g., 0° measurement) are cancelled out by performing a combined measurement at two separate orientations, e.g., 0° and 180°. This principle will be demonstrated later on bellow. Therefore, NFS measurements of the true north are considered much more accurate compared to the gyro-compassing method. However, a high quality stage-based NFS is very expensive—its price is comparable to the price of an entire INS device. Therefore, even though the measurement of the initial conditions by stage-based NFS is more accurate, it is rarely used with typical INS devices, in view of the cost.
As noted above, it is extremely important to determine the initial conditions as accurately as possible. A substantial drawback of gyro-compassing (GC) process of the prior art is that the system is required to be static during a prolonged period of time. Such relatively long period could not be shortened in the prior art without harming the accuracy, notwithstanding of the use of high-quality and expensive IMU.
As also noted above, the IMU of a typical gyro compassing based INS of the prior art are installed on a stationary platform. Therefore, in order to obtain the initial conditions with high accuracy, the use of very accurate and expensive IMU is required, and still this process takes a relatively long duration of several minutes. The accuracy of the gyro-compassing north finding determination is typically a function of the grade of components as well as of the period of measurement. Typically, the longer the process period, a higher accuracy is obtained for a given IMU.
In view of the above, gyro-compassing can only be performed if the IMU of the INS is very accurate, and hence gyro-compassing is traditionally limited to large and expensive navigation systems, this being one of the main drawbacks of the gyro-compassing procedure. As mentioned, once the position of a static system has been initialized from an external source and attitude has been estimated using gyro-compassing, the system is free to move since the inertial sensors of the INS can measure and track any dynamics that the INS may go through. Essentially, the navigation algorithm of the INS numerically integrates the readouts of the inertial sensors in order to continuously update the navigation solution (PVA) and provide it to a host system.
Since the navigation algorithm is basically a numerical integration process, errors in the inputs to the algorithm tend to accumulate during the operation. Once a system is no longer static and changes are measured relative to the initial state, and if left uncompensated, then these errors will grow unboundedly. Indeed, errors in PVA initialization of the algorithm, together with sensor errors during operation, cause deterioration of the performance along time. To summarize, accurate initial conditions and low sensors errors are extremely important to the overall PVA accuracy. To be more specific, errors in the initial conditions are of the major contributors to the total navigation solution errors. If navigation errors cannot be kept bounded by using external references, like the Global Positioning System (GPS), then the importance of accurate initial conditions becomes critical. It is to be noted that the availability of external references is sometimes limited.
The present invention provides initialization of the azimuth in accuracy beyond the accuracy achievable by a standard gyro-compassing procedure. Furthermore, and as will be elaborated hereinafter, by utilizing the north finding (NF) procedure at static periods during mission, the system of the present invention presents a significant reference for the INS algorithms. For the sake of convenience, the system of the invention will be referred to briefly as “stage mounted INS”.
It is an object of the present invention to provide an improved INS system that allows obtaining a more accurate determination of the direction to true north than the one achievable by use of a conventional gyro-compassing process. More specifically, an object of the present invention is to achieve either higher accuracy for a given duration of the process, or to achieve a given accuracy in a shorter process period compared to the prior art gyro-compassing process. Moreover, an object of the present invention is to provide a system which in turn provides a new reference for navigation updates.
It is another object of the invention to provide a system that overcomes the aforementioned drawbacks of the prior art, namely, a system that can be used in an inertial measurement system of a lower grade than required by traditional gyro-compassing based INS.
Other objects and advantages of the invention will become apparent as the description proceeds.